Various proposals have conventionally been made to bring the outer and inner members into their relatively movable states and to retain them stationary.
Such prior art systems include those disclosed in Japanese Patent Publication Nos. 24,290/63; 22,076/70; 29,346/70; and 22,074/73.
In any of means described in these patents, supplying of a high-pressure oil causes one of the outer and inner members to be radially expanded or shrinked toward the other, thereby bringing them into their stationary states, while releasing of the high-pressure oil causes the one member to be shrinked or expanded away from tile other, thereby bringing them back into their relatively movable states. However, if supplying of the high-pressure should be stopped due to any trouble generated in an oil pressure source, both of the members may come into relatively movable states, and there is a fear that relative movement of the members may occurs inadvertently to cause any unexpected accident.
In order to overcome the above problem, a pressure-discharged type retaining system has been proposed and used which is designed so that when a high-pressure fluid is supplied, the outer member is expanded radially, whereby both of the inner and outer members are brought into relatively movable states, and when the high-pressure fluid is discharged, the both members .are put into a close fit and retained stationary. In this system, an oil pressure is applied between a cylinder which is the outer member and a piston which is the inner member, thereby expanding the peripheral wall of the cylinder, so that the piston can be driven, and releasing of the oil pressure from between the cylinder and the piston causes the cylinder to be shrinked to its original state, thereby retaining the piston stationary.
With such a construction, however, in order to ensure the stationary retaining by a close fit, both of the piston and the cylinder are made of a tough material such as a high strength steel, a stainless steel or the like and hence, there is a fear that dragging may be produced between the inner peripheral surface of the cylinder and the outer peripheral surface of the piston, or leakage of the high-pressure oil supplied between these inner and outer peripheral surfaces may occur, or the piston may be accidentally put into a close fit with the cylinder in the course of assembly, with the result that the piston cannot be further inserted into the cylinder.
Therefore, there is a conventionally known system comprising a cylindrical liner interposed between the cylinder and the piston and made of a material such as bronze or the like having a lower hardness than those of the cylinder and the piston, wherein an oil pressure is applied between the outer peripheral surface of the piston and the inner peripheral surface of the cylinder to expand the cylinder along with the liner, so that the piston can be thereby driven. In this system, the piston can be retained stationary in a condition of the oil pressure released and hence, the safety is extremely high. Moreover, since the liner is formed from a relatively soft material, dragging to the piston or the like cannot be produced, and a situation incapable of assembling cannot be also generated during assembling.